1. Field of Invention
The invention relates to the technology associated with an analog to digital conversion, and more particularly to a method for converting an analog signal to a digital signal and a low cost analog to digital converter using the same.
2. Related Art
With the progress of the technology, the electronic technology has been evolved from the earliest vacuum tube and transistor to the integrated circuit which has the quite wide applications. Thus, the electronic products have gradually become the indispensable essentials in the life of the modern human beings. Many products are redeveloped with electronic components such as the electric piano, the electric toothbrush, etc. The purpose of the embedded electronic functions is to let people use the products more conveniently. Hence, lots of novel control systems are developed. Moreover, the analog to digital converter is widely used in the control systems such as the processing of sound, the measurement of temperature, the processing of webcam image and so forth.
FIG. 1 is a circuit diagram showing an analog to digital converter according to the prior art. Referring to FIG. 1, the resolution of the conventional art is 3 bits. The analog to digital converter includes 8 comparators 101-108, 9 resistors R101-R109, a decoding circuit 109 and a voltage follower 110. The voltage follower 110 is used for impedance match and outputs an output voltage which is equal to input voltage VIN. The 9 resistors R101-R109 are connected in series and correspondingly coupled to the negative terminals of the comparators 101-108. The supply voltage VDD is divided into 8 voltage sources by the 9 resistors R101-109. As a result, the negative terminal of comparator 101 receives 1/9 VDD, the negative terminal of comparator 102 receives 2/9 VDD . . . , the negative terminal of comparator 108 receives 8/9 VDD.
For example, the output signals C01-C05 of comparators 101-105 are positive voltage rail when input voltage VIN is more than 5/9 VDD but the output signals C06-C08 of comparators 106-108 are negative voltage rail. Therefore, the digital output values of the decoding circuit 109 D01, D02, and D03 are 1,0, and 1.
From the example, to construct a 3-bits analog to digital converter needs at least 9 resistors R101-R109, 9 comparators 101-108, 110 and a decoding circuit 109. Therefore, to construct an 8-bits analog to digital converter needs at least 257 resistors, 256 comparators, and a 256 to 8 decoding circuit. However, it is not cost effective for the system design manufacturer or integrate circuit manufacturer.